1. Six axenic strains of Blastocystis hominis varied in content of lipids from 12 to 43 pgrams total lipid/cell. With all strains, phospholipid content was about 39% of total lipids. 2. Neutral lipid fractions of B. hominis were resolved into nine constituents, of which seven were identified tentatively. Sterol esters, principally esters of cholesterol, were the major neutral lipid constituent, accounting for 49 to 63% of the neutral lipids, and at least 30% of the total lipids. 3. Polar lipids were resolved into eleven constituents, of which nine were identified tentatively. Phosphatidylcholine was the major polar lipid constituents of all strains, accounting for 53 to 63% of the polar lipids, and about 22% of the total lipids. Progress: The work continues. The thrust now is to determine cholesterol requirement, cholesterol synthesis, if any, and incorporation of 3H- palmitate and 14C-cholesterol. The results will constitute a second publication.